11 1 Thyristor power units JUMO, FAS 620, Edition 02.03 Fig. 3 shows the relationships for a firing angle α = 45°. When α = 0° the output is at a maximum, i.e. the supply voltage is applied to the load without interruptions. On the other end, when α = 180° the voltage is continually blocked by the thyristor through the half-cycle. The dotted line shows the voltage waveform while the thyristor is in the blocking (off) state at a firing angle of 45°. This mode of operation is suitable for resistive, inductive and resistive-inductive loads. In the first case, the load voltage and current are in phase, but in the other cases, the current lags behind the voltage. Thyristor power units from JUMO have a built-in soft-start circuit for transformer loads and a current limiting function, to ensure that no excessively high current flows when the load is switched on for the first time. The phase angle starts at α = 180° (completely cut back) and is then gradually advanced to the required control angle. The advantages of phase-angle control are the fine control of the power output and the fast re- sponse time, which makes it possible to use it for very fast control loops. Current limiting can also be implemented in this way. The disadvantage of this mode of operation is the generation of harmonics by the fast transitions of the cut-back half-cycles of the supply at the firing point and the HF interference that this produces. Another disadvantage is that a reactive power component appears, even when driving a resistive load. With resistive loads this is entirely due to the phase-angle control, and it is therefore known as phase control reactive power. The generation of the phase control reactive power can be understood if one studies the Fourier analysis of the cut-back half-cycles of current. These can be represented by sinewaves of various harmonic frequencies superimposed upon the fundamental frequency. The phase shift of the fun- damental frequency of the current with respect to the load voltage is responsible for the above- mentioned reactive power. 1 Thyristor power units 12 JUMO, FAS 620, Edition 02.03 1.3.2 Burst-firing operation In burst-firing operation, complete sinusoidal cycles of the supply voltage are either switched through to the load or inhibited. In this mode of operation, the power supplied to the load is regu- lated by the proportion of active cycles, and this proportion is determined by a continuous analog signal from a controlling device, such as an electronic controller. The proportion (or duty cycle) is defined as follows (Fig. 4): From this, the power supplied to the load can be derived as P = Pmax · Fig. 4: Burst-firing operation Thyristor power units from JUMO provide the option of choosing between a fixed clock period of 500ms and a variable clock period. In this second option, the thyristor electronics always uses the fastest feasible clock frequency for the output level that is required. For instance, a power level of 50 % can be implemented with one full sinewave cycle of current fol- lowed by one cycle that is left out. Assuming that the supply frequency is 50Hz, the resulting clock frequency will be 25Hz, corresponding to a 40 ms clock period. The operational option with variable clock periods is the one that comes closest to phase-angle control, because of the short pulse bursts. Choosing the variable clock period option means that the thyristor power unit can achieve fine regulation of the output power and yet still respond quick- ly. It is therefore better suited to fast control loops than the option with a fixed clock period. A fixed clock rate is mainly used with transformer loads or in the master-slave economy circuit (Chapter 5.1.2.3). When using power units that operate on the zero-crossing principle, care must be taken that only complete cycles of the sinewave are switched. This is to ensure that there is no resulting DC component, which would cause a very detrimental Duty cycle ON time T e Clock period T = (1) T e T 13 1 Thyristor power units JUMO, FAS 620, Edition 02.03 loading of the supply network or any transformers in the supply feed. Burst-firing has the following advantages over phase-angle control. - Since the thyristors are always switched at the zero-crossing point of the voltage (for a resistive load), the generation of RF interference is minimized. - The load current is purely sinusoidal, so no harmonics are generated. - As long as purely resistive loads are driven, there is no inductive load on the supply, unlike phase-angle control. There is no lagging reactive current to produce a reactive power. The disadvantage lies in the fluctuations in the supply voltage that can result from the clocking of the load when the supply feed does not have a sufficiently low impedance. This effect, known as voltage flicker, causes unpleasant variations in the light intensity, i.e. flickering, of any lighting in- stallations that are fed from the same supply. Limits for this flickering can be found in the European Standard EN 61 000-3-3. The switching at the zero-crossing point leads to the inrush effect in transformers, whereby the iron in the transfomer core becomes magnetically saturated, with the result that the primary current is then effectively only limited by the resistance of the primary winding. In such a case, the current surge at switch-on can reach something like 50 x the rated current. In order to be able to obtain the advantages of pulse-burst operation – such as low reactive power – with transformer loads, burst-firing with cut-back of the first half-cycle can be used as an operat- ing mode (Fig. 5). The phase control angle for the first half-cycle of a pulse group (burst) – also known as α Start – can be set to a value between 0° and 90°, to achieve optimum matching for the particular transformer that is used. Fig. 5: Burst-firing with cut-back of the first half-cycle of the supply voltage 1 Thyristor power units 14 JUMO, FAS 620, Edition 02.03 1.3.3 Burst-firing operation with phase-angle controlled start This type of operation begins with a soft start under phase-angle control. When this has advanced to a complete half-cycle, the controller switches over automatically to burst-firing operation. If the thyristor power unit also includes an automatic current limiting function, it will keep running under phase-angle control up to the point where the automatic changeover to burst-firing opera- tion no longer drives the current above the (adjustable) limit. The starting angle α Start for the first half-cycle of each burst can also be adjusted between 0° and 90° for the subsequent operation of the transformer load. Fig. 6: Burst firing operation with phase-angle controlled start During burst-firing operation, the maximum OFF time is also monitored. If there is a somewhat longer break between bursts, the thyristor power unit falls back into phase-angle control for a fresh soft start. This type of operation is used for transformer loads or resistive loads that have a strongly temperature-dependent resistance (e.g. R cold : R hot ≈ 1 : 16 for Kanthal Super heater elements). 15 JUMO, FAS 620, Edition 02.03 2 IGBT power units 2.1 The IGBT as an electronic switch Fig. 7: Circuit symbol for an IGBT The IGBT (Insulated Gate Bipolar Transistor) behaves like an NPN transistor with an insulated MOSFET gate as the control electrode. The drain of the MOSFET device is accordingly designated as the collector and the source is designated as the emitter. When an IGBT is driven in the forwards direction, the collector-emitter path will become conductive if a positive voltage is applied between gate and emitter. The device is shut off by a negative voltage between gate and emitter, even if a current is flowing between collector and emitter at the time. The IGBT is only used as a switching device, and not as a linear amplifier. An IGBT has a very high voltage blocking capability, and the saturation voltage (the voltage drop between the collector and emitter when the device is conducting) is comparatively low. It is very easy to control through the gate electrode, and the switching losses are acceptable. Gate Collector Emitter 2 IGBT power units 16 JUMO, FAS 620, Edition 02.03 2.2 The IGBT power unit as a control device Fig. 8: Basic circuit of an IGBT power unit The changeover switch shown in Fig. 8 a) can be replaced by a single switch, if a diode is inserted into the circuit as shown in b). While the switch is closed, the current through the choke rises at a rate that is determined by the value of the inductance. When the switch is opened, the current in the choke continues to flow in the same direction, but now through the freewheel diode. During this period the current falls, until the switch is closed once more. So the ON/OFF ratio for the switch determines the waveform of the load current. In practice the switch shown in b) is not a mechanical switch, but a semiconductor power switch- ing device such as, in this case, an IGBT. 17 2 IGBT power units JUMO, FAS 620, Edition 02.03 Fig. 9: Supply voltage/current and load voltage/current for an IGBT power unit The load voltage waveform shown in Fig. 9 b) is an idealized one. In practice, a tolerance band is defined for the intended form of the load voltage waveform and pulse-width modulation is used to keep the actual load voltage within this band. This means that there is a noise signal superimposed on the load voltage, but the level of the harmonics which are produced is relatively low. The IGBT power unit has only one mode of operation – amplitude control. Put simply, this means that the user (system) generates a control signal (for example, a 0 — 20 mA standard signal) for the set level, and the IGBT power unit produces a pulsed DC output that has an amplitude that is pro- portional to this control signal. A pulsed DC voltage with an effective value of 230V is indicated as follows: DC230V . 2 IGBT power units 18 JUMO, FAS 620, Edition 02.03 Note: the output voltage has a DC component, so this circuit must never be used to drive a trans- former load. The JUMO IPC is a power converter for controlling heater loads that previously required a trans- former (either a variable transformer or a combination of transformer + thyristor power unit). It func- tions in such a way that you can think of it as being effectively an electronic transformer with a pulsed DC output voltage. It combines the advantages of the normal variable transformer, such as usual amplitude regulation and sinusoidal supply loading, with the advantages of a thyristor power unit, such as current limit- ing, load monitoring, underlying control loops and so on. There is no electrical isolation between the (input) supply voltage and the (output) load voltage. This converter is suitable for all those applications where substantial resistive loads have to be switched. Thanks to the amplitude control (which means that the current drawn from the supply is always sinusoidal), synchronous clock controls (as for burst-firing operation) or power-factor com- pensation networks (to compensate for phase control reactive power) are not required. Fig. 10: Block structure 19 2 IGBT power units JUMO, FAS 620, Edition 02.03 Special features of IGBT power units: - Low interference (flicker) on the electrical supply when operating substantial resistive loads - Can operate low-voltage heater elements directly from the electrical supply, without needing a step-down transformer - Minimum harmonic generation in the supply to the equipment, and low weight (no power transformer needed) - Short-circuit proof during power-on - Supply current drawn is proportional to the power required (amplitude control) - Control is independent of the resistance characteristic of the heater elements - Compensation for the ageing process in SIC heater elements - Minimum control reactive power - Compact dimensions - Free selection of the underlying control loop: V 2 , P, I 2 2 IGBT power units 20 JUMO, FAS 620, Edition 02.03 . acceptable. Gate Collector Emitter 2 IGBT power units 16 JUMO, FAS 620 , Edition 02. 03 2. 2 The IGBT power unit as a control device Fig. 8: Basic circuit of an IGBT power unit The changeover switch. elements - Minimum control reactive power - Compact dimensions - Free selection of the underlying control loop: V 2 , P, I 2 2 IGBT power units 20 JUMO, FAS 620 , Edition 02. 03 . load voltage is responsible for the above- mentioned reactive power. 1 Thyristor power units 12 JUMO, FAS 620 , Edition 02. 03 1.3 .2 Burst-firing operation In burst-firing operation, complete sinusoidal